1. Field of the Invention
The present invention relates generally to electron spin resonance spectrometry, and, more particularly, to pulsed electron spin resonance spectrometry at high microwave frequencies in excess of 80 GHz.
2. Description of Related Art
It is generally known to carry out pulsed electron spin resonance experiments at high microwave frequencies cf. "High-Frequency Pulsed Electron Spin Resonance" by J. Allgeier et al., in L. Kevan et al. (Ed.) "MODERN PULSED AND CONTINUOUS-WAVE ELECTRON SPIN RESONANCE", John Wiley & Sons, New York, 1989, pp. 267 to 283; "ELECTRON SPIN ECHO SPECTROSCOPY AT 95 GHZ" by R. T. Weber et al., in C. P. Keijzers et al. (Ed.) "Pulsed EPR: A new field of applications", North Holland Publishers, Amsterdam, 1989, pp. 186 to 190.
Electron spin resonance spectroscopy is an analytical measurement technology having been known since about 50 years and having been well-established in physics, chemistry and biochemistry. Although electron spin resonance and nuclear magnetic resonance are based on the same elementary physical phenomena, the development in the respective spectrometer technology has taken a very different course in the two fields.
For example, the gyromagnetic ratio of electrons is about a factor of 1000 higher than that of nuclei (protons). Therefore, classical apparatuses for electron spin resonance spectrometry work at a field strength of 0.3 Tesla and a measuring frequency at X-band (e.g. 9.6 GHz), whereas nuclear magnetic resonance experiments have been carried out at fields of 1.4 Tesla field strength and at measuring frequencies of 100 MHz. The frequency ranges and magnetic field strength ranges were, therefore, distinct from each other by about one order of magnitude.
Due to the development of superconducting magnets the measuring frequencies have been shifted upwardly in the two fields. Therefore, nuclear magnetic resonance experiments are today carried out at measuring frequencies of up to 750 MHz, the magnetic field strength being in the order of 10 Tesla.
In electron spin resonance spectroscopy, too, measurements have been carried out at substantially higher measuring frequencies as mentioned above, e.g. at 95 GHz or even at 140 GHz; "A. Yu. Bresgunov Pulsed EPR in 2-mm Band", Applied Magnetic Resonance 2, pp. 715 to 728 (1991).
Electron spin resonance experiments have been carried out until about 10 years ago exclusively as continuous wave experiments. However, since that time the technology of pulsed electron spin resonance has also been developed. A typical electron spin resonance spectrometer for pulsed experiments is, for example, disclosed in U.S. patent specification 4,812,763 (Schmalbein).
In conventional pulsed electron spin resonance experiments using a field of, for example, 0.35 Tesla field strength microwave pulses having a pulse length of 10 ns at a microwave power of 1 kW are required for achieving the flip angle of electron spins. The necessary resolution during receiving the pulsed signals is in the order of 10.sup.-9 sec. Under these conditions the information that may be expected to be provided by the experiment, in particular the resolution, is in the order of nuclear magnetic resonance measurements with a measuring frequency of 14 MHz, related to protons (.sup.1 H).
As may be taken from the aforementioned considerations, the technology of pulsed electron spin resonance spectroscopy was pretty soon limited by the available technology because electronic components, in particular microwave components in the nanosecond range, still more particularly fast switching microwave switches, were hardly available on the non-military market. However, at measuring frequencies at X-band such components are meanwhile also available on the civil market so that the technology of pulsed electron spin resonance spectrometry has in the meanwhile also been established at this measuring frequency.
On the other hand, with all measuring technologies of magnetic resonance one wishes to increase the measuring frequency to the best possible extent because the measuring sensitivity increases with the square of the measuring frequency.
Therefore, as mentioned above, one has already made attempts to shift the measuring frequency of pulsed electron spin resonance experiments into the range of above 100 GHz.
In the scientific literature of Allgeier and Weber, mentioned at the outset, a pulsed electron spin resonance spectrometer of insofar identical design is described as having an effective measuring frequency of 94.9 GHz.
For generating the measuring frequency in this prior art spectrometer a first microwave oscillator is used having a frequency of 91.9 GHz and an output power of 150 mW. The output signal of this first microwave oscillator is mixed in a mixer with the signal of a second microwave oscillator having a much lower frequency of 3 GHz. For technical reasons the output signal from the second microwave oscillator is gated with a gate time of 20 .mu.s. The gated microwave signal of 3 GHz is now mixed in a mixer with the 91.9 GHz signal from the first microwave oscillator. Because of the mixing a sum signal of 94.9 GHz is generated, however, the output power of the mixer is only 4 mW due to the overall losses in the various components. Therefore, the 94.9 GHz signal is fed into an injection-lock-amplifier for raising the signal level again up to 210 mW.
In order to generate the very narrow and highly dynamic pulses in the nanosecond range as required for pulsed experiments, the prior art spectrometer uses a series arrangement of 3 PIN-diodes being controlled from a central computer by means of separate drive units. The PIN-diodes, therefore, are switched in the circuit path where the microwave signal of high frequency (94.9 GHz) has a high power (210 mW).
The pulsed microwave signal so generated is then fed, as known per se, to the measuring resonator via a circulator. The reflected signal from the resonator, emanating from the circulator is then fed to a further mixer having the original output signal of the first microwave oscillator of 91.9 GHz applied to its second input such that a mixed output signal of again 3 GHz is generated. The 3 GHz signal is then mixed in a subsequent mixer with a 3 GHz signal from the second microwave oscillator.
The prior art spectrometer has several drawbacks. The main drawback is that PIN-diodes having an operating frequency of about 95 GHz are not yet available as standard components and, therefore, are available in the non-military area only at extreme costs. Moreover, such components are extremely sensitive and may even be destroyed if simply touched with the bare hand. Moreover, even if available, such components have a very high input attenuation and a poor isolation. Even if, as discussed with the prior art spectrometer, three PIN-diodes are switched in series, the switching behaviour is not sufficient and the input attenuation may be up to -20 dB. Therefore, with the prior art spectrometer one has attempted to compensate for such high input attenuation by using an injection-lock-amplifier ahead of the diodes. However, due to its concept, an injection-lock-amplifier may only be operated in a pulsed mode with the gated signals as already mentioned. Therefore, additional timing circuitry is necessary for switching the PIN-diodes such that the pulsed nanosecond signals which are needed for the pulsed experiments are correctly placed within the gated microsecond amplifier output signal. However, this is a non-acceptable limitation for pulsed electron spin resonance experiments because it is mandatory to be able in such experiments to place the microwave pulses at arbitrary times for making dead times as small as possible. This holds true in particular when the free induction decay (FID) is being observed when the pulse is still applied.
Finally, the prior art spectrometer has the drawback that even in the lower frequency section a specialized configuration is necessary, not enabling to alternately carry out conventional pulsed or CW experiments at low microwave frequencies without the necessity of entirely reconstructing the spectrometer.
In the other prior art spectrometer as described in the scientific literature of Bresgunov, mentioned before, a microwave measuring signal of 140 GHz is transmitted over a total of four circulators, two of which being provided with PIN-diodes.
Therefore, this prior art spectrometer, too, has the drawback that the switching elements are arranged in the signal path of high frequency.
It is, therefore, an object underlying the present invention to improve a method and a spectrometer such that a measuring frequency in the order of 100 GHz or above may be used although conventional, reliable and easily available components may be used as are already standard for pulsed experiments and which are designed for lower microwave frequencies, typically for X-band.